A polymer porous material having numerous fine continuous pores is utilized in various fields including a separation membrane used for production of ultrapure water, purification of a drug solution, water processing and the like, a moisture permeable waterproof film used for clothing, a sanitary material and the like, and a battery separator used in a secondary battery and the like.
A secondary battery has been widely used as a power source for an office automation equipment, a factory automation equipment, a home electronic equipment, and a portable equipment, such as a communication equipment. In particular, there has been increasing use of a portable equipment using a lithium ion secondary battery, which leads to reduction in size and weight of the equipment due to the good volume efficiency thereof on installing in the equipment. A large-scale secondary battery has been studied and developed in many fields relating to energy and environmental issues, such as a load bearing equipment, an uninterruptible power supply system (UPS), and an electric automobile, and there have been spreading applications of a lithium ion secondary battery, which is a kind of a nonaqueous electrolytic solution secondary battery, due to the large capacity, the high output power, the high voltage, and the excellent long-term storage stability thereof.
A lithium ion secondary battery is generally designed to have an upper limit of the working voltage of from 4.1 to 4.2 V. An aqueous solution cannot be used as an electrolytic solution therefor since an aqueous solution undergoes electrolysis at such a high voltage. Accordingly, a so-called nonaqueous electrolytic solution using an organic solvent is used as an electrolytic solution that is capable of withstanding the high voltage. As a solvent for the nonaqueous electrolytic solution, an organic solvent having a high dielectric constant capable of retaining a larger amount of lithium ion therein is used, and as the organic solvent having a high dielectric constant, an organic carbonate ester compound, such as propylene carbonate and ethylene carbonate, is mainly used. The solvent is used such that a highly reactive electrolyte, such as lithium hexafluorophosphate, as a supporting electrolyte functioning as a lithium ion source, is dissolved in the solvent.
A lithium ion secondary battery has a separator intervening between the positive electrode and the negative electrode for preventing internal short circuit from occurring. The separator is naturally demanded to have insulating property due to the function thereof. The separator is also demanded to have permeability as a path for lithium ion and a fine porous structure for performing diffusion and retention of an electrolytic solution. A porous film is used as the separator for satisfying these demands.
The importance of safety of a battery is being increased associated with the increase of the capacity of the battery in recent years. The characteristics contributing to the safety of the battery separator include shutdown characteristics (which are hereinafter referred to as SD characteristics). The SD characteristics relate to such a function that the fine pores of the porous film are closed in a high temperature state of approximately from 100 to 150° C., and as a result, the ionic conduction of the inside of the battery is interrupted, thereby preventing the temperature of the inside of the battery from being increased after that. In this case, the lowest temperature among the temperatures, at which the fine pores of the porous film are closed, is designated as the shutdown temperature (which is hereinafter referred to as a SD temperature). In the case where the porous film is used as a battery separator, the film necessarily has the SD characteristics.
However, associated with the increase of the energy density and the capacity of the lithium ion secondary battery in recent years, there is a possibility of an accident that the normal SD characteristics is not sufficiently exhibited, and the temperature of the inside of the battery is increased beyond approximately 130° C., which is the melting point of polyethylene used as the material of the battery separator, thereby causing the breakage of the separator through heat shrinkage thereof to cause a short-circuit between the electrodes and lead to ignition. Under the circumstances, the separator is demanded to have higher heat resistance than the current SD characteristics for ensuring the safety.
As a porous film used as the separator, for example, there is a proposal of a multilayer porous film containing a polyolefin resin porous film having on at least one surface thereof a porous coated layer containing a metal oxide, such as alumina, and a resin binder (PTLs 1 to 5). The coated layer is formed, for example, by coating and drying a slurry prepared by mixing alumina, a resin binder, and other components.
PTL 6 describes a slurry that contains inorganic oxide powder, such as α-alumina, satisfying the particular condition, a binder, and a solvent, and is for forming a porous film having insulating property on a surface of at least one of a positive electrode, a negative electrode, and a separator constituting a lithium ion secondary battery.
As a preparation method of a slurry containing alumina, PTL 7 describes a wet pulverization method for powder, such as alumina, by multi-stage pulverization with pulverization media having decreasing medium diameters used in sequence for providing a stable slurry with less change in viscosity even with the use of a small amount of dispersant, in which preliminary pulverization is performed in advance separately from the multi-stage pulverization, and in the multi-stage pulverization, the pulverization medium is switched to one having a smaller diameter at the prescribed timing.
PTL 8 describes a production method of an alumina organic solvent dispersion liquid excellent in dispersion stability with various organic solvents, in which metallic aluminum or a hydrolyzable aluminum compound is hydrolyzed in an organic solvent, and deflocculated in the presence of an acid to provide an alumina organic solvent dispersion liquid. In this method, metallic aluminum or a hydrolyzable aluminum compound is hydrolyzed with water in an amount of from 4 to 10 times by mol the amount of the metallic aluminum or the hydrolyzable aluminum compound to provide an alumina slurry, which is deflocculated in the presence of an organic sulfonic acid in an amount of from 0.01 to 0.2 time by mol the amount of the metallic aluminum or the hydrolyzable aluminum compound.